Twelve-pulse rectifiers are widely used to convert alternating current (AC) to direct current (DC). Twelve-pulse rectifiers can draw AC input current with low Total Harmonic Distortion (THD). As is well known to those having skill in the art, a twelve-pulse rectifier generally is produced by coupling the outputs of two six-pulse rectifiers. The two six-pulse rectifiers may be connected via Y-.DELTA. and Y--Y isolation transformers that are 30 degrees out-of-phase. Each six-pulse rectifier may include a rectifier bridge that is arranged in a secondary circuit of a 3-phase transformer.
The design and operation of six- and twelve-pulse rectifiers are described in the following publications: P. C. Sen, Evaluation of Control Schemes for Thyristor-Controlled DC Motors, IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI125, No. 3, August, 1978, pp. 247-255; M. H. Rashid and M. Aboudina, Analysis of Forced-Commutated Techniques for AC-DC Converters, First European Conference on Power Electronics and Applications, Brussels, Oct. 16-18, 1985, pp. 2.263-2.266; and M. H. Rashid and A. J. Maswood, Analysis of 3-Phase AC-DC Converters Under Unbalanced Supply Conditions, IEEE Industry Applications Conference Record, 1985, pp. 1190-1194. The design and operation of six- and twelve-pulse rectifiers also are described in U.S. Pat. No. 4,164,013 to Kaufhold entitled Six-Pulse Rectifier Circuit and U.S. Pat. No. 5,903,066 to Enjeti entitled Active Interphase Reactor for 12-Pulse Rectifier, respectively. Accordingly, the design and operation of six- and twelve-pulse rectifiers need not be described in detail herein.
Twelve-pulse rectifiers also generally include a controller that controls the operation of the twelve-pulse rectifier. The controller also preferably controls the current balance between the two six-pulse rectifiers so that the difference between the currents in the first and second six-pulse rectifiers may be controlled and preferably minimized. For example, as described in a publication entitled Twelve-Pulse Converter With Auto-Balance and Integrated Magnetics Cuts THD by Bourbeau, PCIM November 1999, pg. 94, a controller with an "auto-balance" feature can reduce the difference in the two currents to typically 10 percent or less of the total DC output current.
Other controllers may provide load-sharing between the 30 degree out-of-phase transformers that constitute the twelve-pulse rectifier using "droop control". See, for example, the publication entitled Fault Tolerant Power Supply System Uses the Droop Method of Current Sharing by Frank DiJoseph, PCIM Magazine, 1998. Load sharing between two 30 degree out-of-phase transformers also may be achieved using analog signal comparison.
Unfortunately, the controller may be a source of reliability problems for twelve-pulse rectifiers because the twelve-pulse rectifier generally ceases to operate upon failure of the controller. Moreover, conventional controllers may not be able to balance the currents between the two six-pulse rectifiers sufficiently to reduce the input current total harmonic distortion as desired for high performance applications.